in the Peruvian upwelling regime

T. Fischer, A. Kock, D.L. Arévalo-Martínez, M. Dengler, P. Brandt, H.W. Bange

Bias of oceanic N ² O emission estimates

by multi-day near-surface stratification

in the Peruvian upwelling regime

BMBF-Verbundvorhaben

SOPRAN

RD 1 Ocean Circulation and Climate Dynamics Düsternbrooker Weg 20

D-24105 Kiel

contact: tfischer@geomar.de www.geomar.de

Measuring nitrous oxide (N ₂ O) in the top 10 meters of the Peruvian upwelling

Near-surface N ₂ O gradients exist - associated with shallow nighttime stratification

Existence of multi-day near-surface stratification is verified by glider surveys

References

Calleja, M. Ll., Duarte, C. M., Vaquer-Sunyer, R., Agusti, S., and Herndl, G. J. (2013): Prevalence of strong vertical CO2 and O2 varia- bility in the top meters of the ocean, Global Biogeochem. Cycles, 27, 941-949, doi: 10.1002/bgc.20081 --- Kock, A., Schafstall, J., Dengler, M., Brandt, P., and Bange. H.W. (2012): Sea-to-air and diapycnal nitrous oxide fluxes in the eastern tropical North Atlantic Ocean, Biogeosciences, 9, 957-964 --- Soloviev, A., Edson, J., McGillis, W., Schluessel, P., and Wanninkhof, R. (2002): Fine thermohaline structure and gas-exchange in the near-surface layer of the ocean during GasEx-98, in: Donelan, M.A., Drennan, W.M., Saltzman, E.S., and Wanninkhof, R. (eds.): Gas transfer at water surfaces, AGU Geophysical Mono- graph Series 127, Washington D.C., 181-185 ---

Acknowledgements

This study was supported by the German Federal Ministry of Education and Research through the joint project SOPRAN (Surface Processes in the Anthropocene) under grant no. SOPRAN II FKZ 03F0611A and SOPRAN III FKZ 03F0662A. ---- The friendly support of all crew members of research vessel METEOR during cruise M91 is highly appreciated. Particular thanks to Rudi Link for helping to construct the sampling equipment, and to the bravehearted who sampled N₂O during long hours in the dinghy, drifting in the waves while sacrificing their health. Thanks to Gerd Krahmann who proces- sed the glider hydrographic data. The daily ASCAT global wind field data were provided by A. Bentamy and D. Croize-Fillon of Ifremer, France.

A 1-D model constrained by the glider timeseries can reproduce the N 2 O gradients

Conclusion: Not just diurnal but multi-day stratification seems the necessary condition here causing considerable near-surface N ₂ O gradients and bias of emission estimates.

0 6 12 18 24

10⁻⁶ 10⁻⁵ 10⁻⁴ 10⁻³

local hour

N2

< 15 15 − 60 > 60

−0.6

−0.4

−0.2 0 0.2 0.4 0.6

nmol/kg log 10 conc ( 5 m ) / conc ( 10 m )

N

2

O conc en tr ation wat er depth _{5000 m 3000 m 150 m 100 m} Stratification of top 10m vs. time of day

for regions of high and low N

2

O concentrations.

profiles with N₂O > 15 nmol/kg

at 5m

profiles with N2O < 15 nmol/kg

at 5m

Stronger N ₂ O gradients are associated with higher N ₂ O concentrations and night time stratification

Ratio of N

2

O at 5m and N

2

O at 10m from 45 CTD casts

Do we estimate gas emissions from adequate concentrations ? Motivation

Shallow sampling away from ship‘s influence

N ₂ O measurements during Meteor cruise M91 in December 2012

0.1 - 1 m pump

1 - 10 m Niskin

Vertical concentration gradients in top layer exist and vary regionally.

Shape of concentration profiles resembles density profiles at night.

Glider fleet in Jan/Feb 2013 Regional stratification timeseries with different grades of multi-day stratification

Simple 1-D two layer model Example run: Region I - multi-day stratification causes distinct N ₂ O gradient Bias of emission estimate

if sampling at indicated depth

model vs. observations

Exchange

across the stra- tified barrier layer is only via entrainment.

For the vertical movement of the barrier the observed HLD timeseries are used.

- strong multi-day stratification - episodes of MDS - frequent mixing - diurnal

str atifica tion timeser ies HLD and wind timeser ies

local days

depth in m

5 10 15 20 25 30 35

0 5 10 15 20

local days

5 10 15 20 25 30

0 5 10 15 20

local days

5 10 15 20

0 5 10 15 20

local days

5 10

0 5 10 15

20 −6

−5

−4

−3

5 10 15 20 25 30 35

−10

−5 0 5 10

local days

depth in m wind in m/s 5 10 15 20 25 30−10

−5 0 5 10

local days

5 10 15 20

−10

−5 0 5 10

local days

5 10

−10

−5 0 5 10

local days

log

₁₀

N

²

log

₁₀

N

²

log

₁₀

N

²

log

₁₀

N

²

In total 250 glider days were perfor- med by 8 gliders in 4 main regions, recording hydrography.

local days

depth in m

model N₂O concentrations in nmol/kg

5 10 15 20 25 30 35

2 4 6 8

10 10

20 30 40 50 60 70 80

0 20 40 60 80 100

0 50 100 150 200

nmol/kg

0.5 m

0 20 40 60 80 100

0 50 100 150 200

nmol/kg

10 m

Resulting concentration distributions

50

77

median

median

outgassing

continuous supply

stratified barrier moving vertically

78^oW 77^oW 76^oW 14^oS

13^oS 12^oS

I II

III IV

Extracted composite hydrographic timeseries:

I II III IV

37 days 31 days 22 days 10 days

The stratified layer is extremely low in mixing

N

²

of O(10

^-3

); dissipation rate of O(10

^-8

):

vertical exchange coefficient K of O(10

^-6

).

80^oW 79^oW 78^oW 77^oW 76^oW 15^oS

14^oS 13^oS 12^oS 11^oS 10^oS 9^oS

A

B C

D

−20 0 20 40 60 80 0

2

4

6

8

10

percent

depth in m

I II

III

−20 0 20 40 60 80 0

2

4

6

8

10

percent

depth in m

B A

C D IV

Top 1 meter:

barely any gradient

(median values of bias shown)

N ²

ε

B D A C

I II III IV

0.95 1 1.05 0

0.2 0.4 0.6 0.8 1 1.2

relative N₂O concentration

depth in m

A B C D

Conc. at 10 cm : Conc. at 1 m ≈ 0.97

1 2 3 5 10 20 30

0 50 100 150 200 250 300

emission overestimated by factor ...

Resulting bias factor of emission ( nearly independent of

N₂O supply flux from below )

I II III IV

model forcing timeseries

A B C D

sampled N

₂

O profiles Nighttime

stratification N2 = 10-6 N2 = 10-4 N2 = 7 10-4 N2 = 10-3

In the Maurita- nian upwelling regime, N

₂

O supply from below is much lower than the calculated N

₂

O emissions.

[Kock et al., 2012]